Set pose position to armature
It appears, that you want to apply your new pose to the model as well as the armature.

To do that, make sure you are in the desired new rest pose. Then apply the armature modifier on the object first. The object will now be unlinked from the armature, but it will be in the target pose.

Now, you can apply the Pose as Rest Pose to the armature. Following this, both (armature and mesh) have the desired pose as the default pose.

To clean up, re-add an armature modifier to the mesh and select the armature as the target object of the modifier.